One major example of this is Steam. While most games on Steam have ESRB ratings, some not (such as Valve's own Dota 2).
Is this considered legal?
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According to Wikipedia:
The ESRB ratings system is enforced via the voluntary leverage of the North American video game and retail industries for physical releases; most stores require customers to present photo identification when purchasing games carrying the ESRB's highest age ratings, and do not stock games which have not been rated. Additionally, major console manufacturers will not license games for their systems unless they carry ESRB ratings, while console manufacturers and most stores will refuse to stock games that the ESRB has rated as being appropriate for adults only.
"Ennforcement" is done by the market; it is harder to sell games without ratings. In Valve's case, there is little incentive to rate their games, because they control the entire distribution system. The only reason for a rating would be that some customers may choose not to purchase or play games without ratings; apparently, these customers make up a small enough fraction of the market that Valve is not concerned.
In fact, it is unconstitutional to require ratings, or at least to restrict sale based on ratings. See Brown v. Entertainment Merchants Association.
All computer games must be classified by the Federal government before they can be sold physically or online, although what classifications are allowed or prohibited are a matter for each state or territory.
Since 2014, classification for computer games is self-assessed and the government acts on complaints. Naturally, there’s a lot of non-compliance, especially by indie developers.
The Australian Classification Board does get involved: in 2022, Rimworld was initially Refused Clasification (RC) meaning it could not be sold in Australia, however, it was given a Restricted R18+ on review. The Australian classification can be clearly seen on Steam’s Australian page for the game: